1. Field of the Invention
This invention relates a method for producing alkyl-substituted styrene, preferably p-alkylstyrene, having an alkyl substituent group with 3 or more carbon atoms from monoalkylbenzene through 1,1-bis(p-alkylphenyl)ethane.
2. Description of the Prior Art
It is well known that 1,1-bis(p-alkylphenyl)ethane can be converted into p-alkylstyrene in a high yield by means of catalytic cracking. There are hitherto proposed several methods to synthesize 1,1-bis(p-alkylphenyl)ethane. As one of them, there is proposed a method that p-isobutylbenzene is reacted with acetaldehyde in the presence of sulfuric acid catalyst to obtain 1,1-bis(p-isobutylphenyl)ethane (U.S. Pat. No. 4,694,100).
When isobutylbenzene is used as a monoalkylbenzene as described in the above patent specifications, 1,1-bis(p-isobutylphenyl)ethane is obtained, which is especially used as an intermediate compound for economically preparing ibuprofen (tradename) that is effective as a medicine for the relief of inflammation.
As described in the above patent specifications, however, the sulfonation of valuable isobutylbenzene itself cannot be avoided owing to the use of sulfuric acid. As a result, a part of isobutylbenzene is lost in the form of sulfonation product, which is not desirable in view of economy.
Furthermore, because this reaction is dehydration, the concentration of sulfuric acid as a catalyst is lowered with the progress of reaction due to released water. Therefore, in order to reuse the sulfuric acid, it is necessary to recover the concentration of used sulfuric acid by a method such as high-temperature distillation in which the corrosion of apparatus is apprehended. In addition, the recovery of the catalyst concentration is not easy by a measure of mere distillation because much sulfonation product is dissolved in the sulfuric acid phase.
More particularly, it is known that p-alkylstyrene is obtained in a good yield by catalytically cracking symmetric 1,1-bis(p-alkylphenyl)ethane (foregoing United States Patent). However, it has been difficult to obtain inexpensively the raw material of 1,1-bis(p-alkylphenyl)ethane.
For example, in a method to prepare 1,1-bis(p-alkylphenyl)ethane by reacting monoalkylbenzene with acetaldehyde in the presence of concentrated sulfuric acid, monoalkylbenzene sulfonic acid that is the sulfonation product of the starting alkylbenzene is produced by side reaction. Therefore, the loss of the starting alkylbenzene is not negligible. Furthermore, for the reuse of sulfuric acid catalyst containing much organic sulfonic acid, the concentration of sulfuric acid which was lowered by the generation of water during the reaction must be recovered. However, the method such as distillation in which heat is applied cannot be adopted because the corrosion by hot sulfuric acid is severe. Accordingly, it was necessary to remove the generated water by chemical reaction with adding sulfuric anhydride or fuming sulfuric acid, which increased the cost of catalyst.
On the other hand, when monoalkylbenzene is reacted with acetaldehyde in the presence of hydrogen fluoride catalyst, it is desirable that the side reaction to produce sulfonated product does not occur. However, in accordance with the experiments of the present inventors, when monoalkylbenzene was condensed in the presence of hydrogen fluoride catalyst, it was confirmed that a by-product of p-monoalkylethylbenzene was produced, which is of course undesirable because the by-product causes the loss of material. It is, therefore, required to reduce as far as possible both the loss of raw materials and the side reaction to form p-alkylethylbenzene.
The carbon number of alkyl group in the by-produced p-monoalkylethylbenzene is the same as the carbon number of alkyl group in the starting material of alkylbenzene.
Incidentally, in the case that the number of carbon atoms of the alkyl groups of the by-product of alkylethylbenzenes is less than 3, the dehydrogenated products obtained by dehydrogenating these compounds are all alkylstyrenes, which are nothing but the aimed compounds in the present invention.
Accordingly, if the by-product is dehydrogenated, it may be used effectively and there occurs no disadvantage when an alkylbenzene having an alkyl group with less than 3 carbon atoms is used as a raw material. In addition, the dehydrogenation can be carried out easily.
From such a viewpoint, when a monoalkylbenzene having a substituent chain with less than 3 carbon atoms is used as a starting material for preparing alkylstyrene, there is no use in paying consideration to the generation of the above-mentioned by-product. The reason is that the by-product of alkylethylbenzene such as diethylbenzene and methylethylbenzene can be easily dehydrogenated into the aimed compound of alkylstyrene, and therefore, the by-product is by no means the loss in process.
On the other hand, when the alkylethylbenzene as a by-product which is obtained by using a starting material of monoalkylbenzene having a substituent group with 3 or more carbon atoms, is dehydrogenated, there is a possibility that the alkyl group with 3 or more carbon atoms as well as ethyl group of the alkylethylbenzene are dehydrogenated together. Accordingly, the dehydrogenation products are naturally a mixture of plural kinds of styrene derivatives. What is worse, they cannot be separated easily by ordinary distillation because the molecular weights of them are close to one another. Therefore, the effective use of the by-product of alkylethylbenzene is not possible, which reduces the yield of alkylstyrene corresponding to the quantity of the formed by-product.
In view of the effective use of the by-product, the use of a starting material of such an alkylbenzene having a substituent group with 3 or more carbon atoms, is not desirable. It is, therefore, necessary to suppress the formation of the by-product of this kind.
For the above reason, when a starting material of monoalkylbenzene having a substituent group with 3 or more carbon atoms is used, it is especially necessary to suppress the formation of the by-product of alkylethylbenzene.
In U.S. Pat. No. 3,002,034, a method of reaction with hydrogen fluoride catalyst is referred to. However, in all the reactions described in examples, a raw material of toluene and a catalyst of sulfuric acid are used, while any practical investigation on the reaction using hydrogen fluoride is not made. In view of the description that the reaction temperature is 5.degree. C. or above, preferably 15.degree. to 60.degree. C., the disclosure basically relates to the art to use sulfuric acid as a catalyst. Therefore, the reference teaches nothing with respect to the present invention. In addition, it is described in Example IV of the same reference that the by-product is a high boiling material which is believed to be tetramethyldihydroanthracene and, in Example V to VII, the formation of high boiling polymer is recognized. These examples are all carried out with sulfuric acid catalyst. From these facts, it can be naturally supposed that the kinds and quantities of by-products are varied according to the kind of catalyst.
It is, therefore, the object of the present invention to produce p-alkylstyrene with avoiding the loss in the form of sulfonation products, to suppress the formation of the by-product of alkylethylbenzene as low as possible, and as a result, to reduce the loss of the raw material of alkylbenzene.